diff --git a/ssh_tools/multissh.sh b/ssh_tools/multissh.sh
index 880af3f..725026e 100755
--- a/ssh_tools/multissh.sh
+++ b/ssh_tools/multissh.sh
@@ -1,6 +1,6 @@
 #!/bin/sh
 #
-# multissh.sh, Copyright © 2013 Matteo Cypriani <mcy@lm7.fr>
+# multissh.sh, Copyright © 2013, 2019 Matteo Cypriani <mcy@lm7.fr>
 # (Formerly named cluster-run.sh)
 #
 # This program is free software. It comes without any warranty, to
@@ -12,48 +12,63 @@
 # Run a command on a list of remote hosts using Parallel SSH.
 
 #set -x
+set -u
+
+# Exit codes
+readonly EXIT_SUCCESS=0
+readonly EXIT_USAGE=127
+readonly EXIT_DEPENDENCY=4
+readonly EXIT_HOSTS_LIST=6
 
 print_usage()
 {
   echo "Usage: $0 [-l login] <hosts_list> <command>"
 }
 
-# Do we have at least a host list and a command?
-if [ $# -lt 2 ] ; then
+bad_usage()
+{
+  err "$@"
+  echo >&2
   print_usage >&2
-  exit 1
-fi
+  exit $EXIT_USAGE
+}
+
+err()
+{
+  printf 'Error! ' >&2
+  printf '%s\n' "$@" >&2
+}
+
+# Do we have at least a host list and a command?
+[ $# -lt 2 ] && bad_usage "Wrong number of arguments."
 
 # Parse the optional arguments
+LOGIN=
 if [ "$1" = "-l" ] ; then
   LOGIN="$2"
   shift
   shift
 
   # Do we still have at least a host list and a command?
-  if [ $# -lt 2 ] ; then
-    print_usage
-    exit 3
-  fi
+  [ $# -lt 2 ] && bad_usage "Wrong number of arguments."
 fi
 
 # Check dependencies
 PSSH=$(command -v parallel-ssh || command -v pssh)
 if [ -z "$PSSH" ] ; then
-  echo "Parallel SSH (pssh) is required for this script to work."
-  exit 4
+  err "Parallel SSH (pssh) is required for this script to work."
+  exit $EXIT_DEPENDENCY
 fi
 
-# Hosts' list file
-if [ -z "$XDG_CONFIG_HOME" ] ; then
-  XDG_CONFIG_HOME="$HOME/.config"
-fi
-HOSTS="$XDG_CONFIG_HOME/ssh_tools/$1.lst"
+# Hosts list file
+[ -z ${XDG_CONFIG_HOME+x} ] && XDG_CONFIG_HOME="$HOME/.config"
+HOSTS_LIST_NAME="$1"
 shift
-echo "Using file '$HOSTS' as hosts' list."
+HOSTS="$XDG_CONFIG_HOME/ssh_tools/${HOSTS_LIST_NAME}.lst"
+echo "Using file '$HOSTS' as hosts list."
 if [ ! -f "$HOSTS" ] ; then
-  echo "The hosts' list file doesn't exist or is not a regular file."
-  exit 2
+  err "The hosts list file doesn't exist or is not a regular file."
+  exit $EXIT_HOSTS_LIST
 fi
 
 # Login